The present invention generally relates to position detection apparatus, and more particularly to apparatus for determining the position of a member that is movable along a defined path of finite length.
It is often desirably to identify the position of a device that is controlled by an actuator or the like. For example, in the controls industry, devices such as valves having a valve stem or valve shaft that is movable by an actuator are used to control the flow of liquids and gases associated with industrial processes of various types. In these applications, it is often desirable to identify, at any given time, the precise position of the movable valve stem or valve shaft. This information allows improved understanding of the process and the control of the process.
A number of prior solutions have been proposed. Optical coding schemes make use of a coded element with opaque and transparent sections to provide digital data inputs to an array of sensors positioned to measure the light passing through the sections. While optical coding devices do not require a mechanical linkage, the optical approach only works well in very clean environments and is therefore not applied in many industrial environments. Linear variable differential transformers (lvdt) can provide very accurate position information, but typically require a mechanical linkage and also generally use relatively high power. Potentiometers or other rotary transducers require a mechanical linkage and also have the disadvantage of a sliding electrical contact which can cause long term reliability issues. Hall effect transducers, as they are currently used, generally require a mechanical linkage.
An improved approach for determining the position of a movable member is disclosed in U.S. Pat. No. 4,698,996 to Kreft et al. Kreft et al. suggest providing a bar magnet on the movable member, which then moves parallel to a plurality of spaced sensors. During a calibration procedure, the bar magnet is moved step-by-step in a direction parallel to the line of sensors in precisely defined length units. When an output voltage of a particular sensor is zero, while neighboring sensors on either side thereof have respective positive and negative values, a length value is assigned to the particular sensor and stored.
For unknown positions of the magnet, the voltage values of neighboring sensors that are influenced by the magnet are measured and the relationships thereof are determined. Adjacent sensors that have voltage values that are of different polarity are selected. For voltage relationships which correspond exactly to a calibrated voltage relationship, the corresponding calibrated positional value is assigned to the unknown position. For voltage relationships lying between the calibration values, suitable interpolation methods are used to define the position of the magnet.
A limitation of Kreft et al. is that no compensation is provided for the nonlinearity of the sensor output signals, or the non-linearity in the magnetic field due to imperfections in the magnet or the like. Kreft et al. recognize that the sensor output signals may be non-linear, particularly when a pole of the magnet approaches the sensor. Thus, to ensure that the neighboring sensors operate in the linear region, Kreft et al. suggest using a long magnet relative to the spacing of the sensors so that the poles of the magnet are sufficiently displaced from both neighboring sensors. This, however, can significantly increase the cost of the position determining device.
Therefore, a need exists for a position determining apparatus that does not require a long magnet and/or small sensor spacing, while still reliably and accurately determining the position of the magnet.